The head of the city teachers’ union declared war today on Mayor Bloomberg’s plan to layoff 4,100 teachers – and angrily vowed a massive Wisconsin-style protest this Thursday as an opening salvo.

“Mr. Mayor, it’s not going to happen, and enough is enough!” shouted Michael Mulgrew, president of the United Federation of Teachers, as he whipped up a roaring crowd at the UFT’s spring conference in Midtown.

A ballroom-full of educators rose to their feet, clapping and chanting, “Enough is Enough.”

Schools Chancellor Dennis Walcott, whom Bloomberg appointed last month after firing Cathie Black, remained seated on the dais and did not clap at Mulgrew’s remarks, though minutes earlier the two shook hands and embraced. Walcott was warmly applauded by the teachers, telling them, “I think the world of you.”

But they gave a standing ovation to a surprise guest, Wisconsin State Sen. Jon Erpenbach, who led 13 fellow lawmakers into hiding out of state to block Gov. Scott Walker’s legislation to revoke union bargaining rights.

The UFT and other city unions plan to draw tens of thousands to a May 12 march from City Hall and other sites to Wall Street to demand that Bloomberg make big banks rescued by the bailout pay more to prevent cuts for education and other city services.

Bloomberg unveiled a $65.7 billion city budget Friday that calls for cutting 6,100 teacher jobs, including 2000 through attrition.

“It’s a political game,” Mulgrew charged. He said the rally will include parents, students, clergy and others in a show of defiance. “We will not stand by and accept this any more.”

Mulgrew hammered Bloomberg for refusing to tap $270 million from a “$3.2 billion surplus” to save teacher jobs.

Walcott said later the surplus is $2 billion, and the mayor tapped savings to “stave off even deeper cuts.” He said some money must be held for “bigger holes” next year and beyond.